A monocrystalline silicon has been manufactured by the Czochralski method (the CZ method) using a vitreous silica crucible. In particular, a silicon melt obtained by melting polycrystalline silicon raw material is retained in the vitreous silica crucible, and then the silicon melt is allowed to come in contact with a seed crystal of a monocrystalline silicon, followed by gradual pulling of the seed crystal while rotating the seed crystal. Thus, the monocrystalline silicon is manufactured by allowing the single crystal to grow with the seed crystal acting as a core. The softening point of the vitreous silica is approximately 1200 to 1300° C., while the pulling temperature of the monocrystalline silicon is 1450 to 1500° C., which is an extremely high temperature above the softening point of the monocrystalline silicon. In addition, the time required for the pulling may exceed two weeks.
The monocrystalline silicon being pulled is required to have a purity of 99.999999999% or higher, and thus the vitreous silica crucible used for the pulling is also required to have high purity.
The diameter size of the vitreous silica crucible varies as 28 inches (approximately 71 cm), 32 inches (approximately 81 cm), 36 inches (approximately 91 cm), 40 inches (approximately 101 cm), and the like. The crucible with the diameter of 101 cm is huge, with its weight being approximately 120 kg. The weight of the silicon melt being retained therein is 900 kg or more. Accordingly, when pulling the monocrystalline silicon, 900 kg or more of the silicone melt at approximately 1500° C. is retained in the crucible.
One example of the manufacturing method of such vitreous silica crucible includes a silica powder layer forming step to form a silica powder layer by depositing the silica powder having an average particle diameter of approximately 300 μm onto the inner surface of a rotating mold; an arc fusing step to form a vitreous silica layer by arc fusing the silica powder layer while depressurizing the silica powder layer from the mold side (hereinafter referred to as a “rotating mold method”).
During the early stage of the arc fusing step, the silica powder layer is strongly depressurized, thereby removing bubbles to form a transparent vitreous silica layer (hereinafter referred to as “transparent layer”). Then, the degree of depressurizing is weakened to form a bubble containing vitreous silica layer (hereinafter referred to as “bubble containing layer”), with remaining bubbles. Accordingly, a vitreous silica crucible having a two-layer structure, with the transparent layer at the inner surface side and the bubble containing layer at the outer surface side, can be formed.
As the silica powder used for the manufacture of the crucible, natural silica powder manufactured by pulverizing natural quartz, and synthetic silica powder manufactured by chemical synthesis can be mentioned. Here, physical property, shape, and size of natural silica powder tend to vary since the raw material of the natural silica powder is a natural product. When the physical property, shape, or size varies, the fused state of the silica powder varies, and thus the three-dimensional shape of the crucible being manufactured would vary even when the arc fusing is conducted under the same conditions.
In order to conduct the arc fusing with an intent to suppress such variation, or to conduct the pulling of the monocrystalline silicon by taking such variation in consideration, it is necessary to understand the three-dimensional shape of all crucibles.
As a method for measuring the three-dimensional shape, a light section method in which an object to be measured is irradiated with a slit light, and a pattern projection method in which the object to be measured is irradiated with a pattern light, are disclosed as conventional techniques in Patent Document 1.
Here, when measuring the three-dimensional shape by the light irradiation method, the data of the three-dimensional shape for the object to be measured is obtained by receiving the reflected light reflected by the object to be measured, and then analyzing the data of the reflected light. Therefore, it is important to receive the appropriate reflected light. However, when the object to be measured is a transparent body such as a vitreous silica crucible for example, the three-dimensional shape may not be measured properly due to the internal scattering light.
Therefore, when measuring a transparent body, a reflecting material such as white powder and the like was applied onto its surface to suppress the generation of internal scattering light.